1. Field of the Invention
The present invention relates to a control method for an ink jet printer whereby characters expressed using dot matrices are printed using a print head that has plural nozzles and moves parallel to the printed line, and relates particularly to a method for generating preliminary ejection data according to which ink is ejected from the nozzles to refresh the print head.
2. Description of the Related Art
In general, conventional ink jet printers print to a recording medium by writing the image signal into memory or other storage means, and then selectively driving pressure-inducing means such as piezoelectric elements or heat-emitting elements proximal to the nozzles based on the image signal.
A nozzle purging process is also commonly used in addition to this printing process. This purging process regularly ejects ink droplets from all nozzles to prevent nozzle clogging and maintain printer performance.
This purging process is referred to as a preliminary ejecting process and is described in Japanese patent laid-open specification number H3-292148 (1991292148). More specifically, Japanese specification H3-292148 describes an ink jet printer comprising a selector for selecting either image data transferred to the printer or the preliminary ejection data generated by a preliminary ejection data generating circuit. This ink jet printer accomplishes either the normal print process or a preliminary ejecting process by writing to memory the data selected according to the selection signal and outputting the stored data to the ink jet head through a head driver.
The problems with this conventional method are described below. (1) The preliminary ejection data generated by the preliminary ejection data generator is written to memory in the same manner as the image data. It is therefore necessary to write the preliminary ejection data to memory for every preliminary ejecting process. This complicates the process and degrades the image data transfer efficiency if the preliminary ejecting process is frequently executed because of the increased frequency of the data transfer interrupts and memory writing operations required to execute each preliminary ejecting process. More specifically, this results in an overall reduction in the printing speed because image data cannot be received by the printer during the preliminary ejecting process. (2) Because the preliminary ejection data generated by the preliminary ejection data generator is written to memory in the same manner as the image data, the printing process is interrupted and the preliminary ejection data is written to memory every time the preliminary ejecting process is executed. This makes it necessary to delete the image data already stored in memory. To then resume the normal printing process after the preliminary ejecting process, the deleted image data must be written to memory again. This again complicates the process, degrades the image data transfer efficiency, and results in an overall reduction in the printing speed. (3) While problems (1) and (2) above can be resolved by increasing the available memory, this also increases the cost. With the popularization of color ink jet printers and high resolution multi-nozzle ink jet printers in recent years, the costs associated with increasing the on-board memory are significant. (4) In order to increase the dot density and thereby increase print quality, ink jet printers having an inclined nozzle row in the scanning direction have been proposed, thereby effectively reducing the vertical pitch between the recording dots. When using an ink jet head in which the nozzles are thus diagonally arrayed, however, both the column print data received by the printer and the preliminary ejection data must be converted to diagonally arrayed print data, thus further degrading the printing speed.